"These drug interactions may be clinically significant for patients infected with both chronic hepatitis C virus and HIV by potentially reducing the effectiveness of these medicines when coadministered," Merck said in a February 6th letter to healthcare professionals.

The drug interactions were seen in a study among healthy volunteers who took Victrelis and the widely used HIV treatment Norvir (ritonavir) in combination with one of three other anti-HIV pills: Reyataz (atazanavir), Prezista (darunavir) and Kaletra (lopinavir/ritonavir).

Victrelis, a hepatitis C protease inhibitor, reduced concentrations in the blood of Reyataz, Prezista and Kaletra by an average of 49%, 59% and 43%, respectively.

Further, levels of Victrelis itself were reduced by 45% among volunteers who took it with Kaletra, and by 32% among those who took it with a combination of Norvir and Prezista.

The U.S. Food and Drug Administration has warned patients not to stop taking any of their medicines without talking to healthcare professionals.